drm/i915: Leave vma intact as they are discarded

If we find ourselves trying to reuse a misplaced but active vma, we
currently try to discard it to avoid having to wait to unbind it
(upsetting the current user fo the vma). An alternative to marking it as
a dicarded vma and keeping it in both the obj->vma.list and
obj->vma.tree, is to simply remove it from the lookup rbtree.

While it remains in the list of vma, it will be unbound under eviction
pressure and freed along with the object. We will never reuse it again
for new instances. As before, with no pruning, the list may continually
grow, but eventually we will have the most constrained version of the
ggtt view that meets all requirements -- so the list of vma should not
grow without bound.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2012


Fixes: 9bdcaa5e ("drm/i915: Discard a misplaced GGTT vma")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200611180421.23262-1-chris@chris-wilson.co.uk